Pulsar timing arrays (PTA) are poised to detect nanohertz-frequency gravitational waves (GWs) within a few years to a decade, opening a window to the low-frequency end of the GW spectrum. The first detection is likely to be a stochastic GW background (GWB) from a population of coalescing supermassive black hole binaries (SMBHBs). Through precision timing of millisecond pulsars (MSPs), PTAs measure differences between the predicted and observed pulse arrival times; after accounting for noise intrinsic to the PTA, the timing residuals will display a correlated low-frequency signal in the presence of a GWB. GW detection with PTAs nominally requires long timing baselines (years to decades) and pulsar timing precision of order 100 ns. Additionally, because of the current sensitivity of PTAs, adding more bright, stable MSPs to the arrays will more rapidly decrease the time to GWB detection, a prime motivation for ongoing pulsar searches. I will describe the process of pulsar timing, pulsar searching, and GW searches in the context of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) PTA, and will discuss preliminary results from NANOGrav's upcoming 11-year data release.

Using High-Mass X-ray Binaries to Probe Massive Binary Evolution

Speaker2:

Kristen Garofali

Institution2:

UW

Abstract

High-mass X-ray binaries (HMXBs) provide an exciting window into the underlying processes of both binary as well as massive star evolution. Because HMXBs are systems containing a compact object accreting from a high-mass star at close orbital separations they are also likely progenitors of gamma-ray bursts and gravitational wave sources. I will present work on the classification and age measurements of HMXBs in M33 using a combination of deep Chandra X-ray imaging, and archival Hubble Space Telescope data. I am able to constrain the ages of the HMXB candidates by fitting the color-magnitude diagrams of the surrounding stars, which yield the star formation histories of the surrounding region. Unlike the age distributions measured for HMXB populations in the Magellenic Clouds, the age distribution for the HMXB population in M33 contains a number of extremely young (<5 Myr) sources, including M33 X-7, an eclipsing binary composed of a ~15 Msun black hole accreting from a 70 Msun O star companion. I will discuss these new results for M33 in the context of the effect of host galaxy properties on the observed HMXB population.